The present invention relates to an apparatus for diagnosing a circuit, and more particularly, to an apparatus for diagnosing a timer circuit that automatically activates a device.
In recent years, in a vehicle such as an automobile, installed devices, such as an engine, a transmission gear, and a brake are controlled by an electronic control unit. An abnormality diagnosis apparatus diagnoses the various devices and the electronic control unit for abnormalities. Such an abnormality diagnosis apparatus is used to improve the reliability of the vehicle. The accuracy of abnormality detection is improved by executing such detection when the condition of the vehicle is stabilized after sufficient time elapses from when the engine is stopped. For example, the abnormality diagnosis apparatus may detect whether a fuel tank has a hole from a pressure change in the fuel tank. In this case, even a subtle pressure change caused by undulation of the fuel liquid level in the fuel tank may influence the accuracy of the abnormality detection. Therefore, to ensure high accuracy in the abnormality detection, the fuel tank is diagnosed when fuel in the fuel tank is stabilized after the engine is stopped.
In a vehicle, however, power supply to equipment including the various devices and an electronic control unit is normally cut after the driver switches off an ignition switch to stop the engine. Since the abnormality diagnosis apparatus is also deactivated, the abnormality diagnosis apparatus cannot execute abnormality detection after the engine is stopped. Therefore, to enable the abnormality diagnosis apparatus to execute abnormality detection after the engine is stopped, an automatic activation timer circuit may be installed in the vehicle to temporarily activate the abnormality diagnosis apparatus (and the device that is to be diagnosed when necessary) after the engine is stopped.
To be more specific, the automatic activation timer circuit bypasses the ignition switch to supply power to devices that must perform or undergo abnormality detection. Further, the automatic activation timer circuit includes a timer unit for measuring the time elapsed from when the power supply is cut by turning off the ignition switch. When the elapsed time measured by the timer unit of the automatic activation timer circuit reaches a predetermined time, the electronic control unit supplies power to the devices that must perform or undergo abnormality detection. As a result, the abnormality diagnosis apparatus is automatically activated temporarily to detect abnormalities in the various devices.
In this case, when the engine is not running, the abnormality detection may be started simply by continuing to supply power to the automatic activation timer circuit. Further, the automatic activation timer circuit, which is provided with the above-described functions, consumes an extremely small amount of power. Thus, the automatic activation timer circuit, when installed in a vehicle, enables abnormality detection to be executed after the engine is stopped, while consuming a small amount of power when the engine is not running.
In the automatic activation timer circuit, however, the timer unit, which functions to measure time, may develop an abnormality. This would result in a fault in the elapsed time measured by the timer unit. To eliminate such a problem, a process for correcting the elapsed time measured by the timer unit is proposed in Japanese Laid-Open Patent Publication No. 2000-329874. In this process, an electronic control unit is supplied with power (activated) whenever a predetermined time elapses, and a timer incorporated in a microcomputer of an electronic control unit is actuated. The time measured by the timer is compared with the elapsed time measured by a timer unit of an automatic activation timer circuit. Based on the comparison result, the elapsed time measured by the timer unit is corrected. Such correction of the elapsed time prevents the occurrence of a fault in the timer unit that would hinder the execution of the abnormality detection at the appropriate timing after the engine is stopped.
Even if an abnormality occurs in the timer unit of the automatic activation timer circuit, the correction of the elapsed time measured by the timer unit as described above would minimize problems caused by such an abnormality. In the automatic activation timer circuit, however, an abnormality may occur in parts other than the timer unit that are related with the execution of automatic activation. For example, an abnormality may occur in a part controlling automatic activation. As another possibility, a wire breakage may occur in a signal line within the automatic activation timer circuit. If such an abnormality occurs, the abnormality diagnosis apparatus may be automatically activated before the elapsed time measured by the timer unit reaches a predetermined time. As another possibility, the abnormality diagnosis apparatus may not be automatically activated even after the elapsed time reaches a predetermined time. In these cases, the abnormality diagnosis apparatus would not be properly activated automatically.
The state of the automatic activation timer circuit must be checked based on whether the abnormality diagnosis apparatus is automatically activated at the timing when the elapsed time measured by the timer unit reaches a predetermined time. Conventional techniques, however, provide no way of checking whether the abnormality diagnosis apparatus is automatically activated at that timing when the predetermined time has elapsed. In other words, conventional techniques fail to determine whether an abnormality relating to the execution of the automatic activation has occurred in the automatic activation timer circuit. Thus, conventional techniques fail to deal with cases in which the automatic activation of the abnormality diagnosis apparatus is not executed correctly because of such an abnormality.
Such problems in an automatic activation timer circuit are not limited to systems used in vehicles, such as automobiles, but may also occur in other systems that are not used in vehicles.